1. Field of the Invention
The present invention relates to an ophthalmic apparatus, and more particularly, to the apparatus for obtaining ophthalmic information having relation to a refractive power and a corneal shape of an eye to be examined, and relates to the apparatus for determining an amount of corneal ablation, which is utilized for correcting ametropia in a manner of ablating a cornea thereby varying its corneal shape, based on the obtained ophthalmic information.
2. Description of Related Art
A refractive power of an eye to be examined, is obtained by using an objective refractive power measuring apparatus, or the like. For example, it is known for this apparatus which projects a measuring target onto a fundus of the eye, then detecting a reflex target image from the fundus thereby obtaining a refractive power. The refractive power reflects the ophthalmic information of a corneal region within 3 mm or less from a corneal center.
A corneal surface shape can be obtained by using a corneal curvature measuring apparatus, or a corneal shape analyzing apparatus. It is known for the corneal curvature measuring apparatus which detects a position of a measuring target being projected onto a cornea, thereby obtaining each curvature in directions of strong and weak principal meridians with defining a corneal shape as a toric. The value of the curvature is only for a corneal region within approximately 3 mm from a corneal center.
It is known for the corneal shape analyzing apparatus which gives an image processing to a numerous placido ring target and the like, projected onto a corneal surface, thereby obtaining distribution of a corneal curvature over a wide region of a cornea. Further, a corneal curvature is expressed in terms of a corneal refractive power. In general, the following expression (*) is used for this calculation. EQU D=(ne-1)/r . . . (*)
Where, r is a corneal curvature, D is a refractive power, and ne is a corneal conversion rate. In general, a value of ne is 1.3375.
The value measured by the refractive power measuring apparatus denotes a refraction amount, i.e. a correcting amount, necessary for causing the eye to be emmetropia. Thus, it differs from the value D obtained from the expression (*) in its true sense. Therefore, the value obtained by each measuring apparatus is handled with under the definition that each value is different one. The relationship among each value is difficult to understand.
Further, it is known for an apparatus for operating upon a cornea, which ablates a corneal surface or its stroma with a laser beam, thus causing a corneal surface shape to vary, thereby correcting ametropia of an eyeball. In this surgical operation, the apparatus obtains a pre-operative corneal shape and its refractive power, thereby calculating an amount of corneal ablation necessary for correction. In the past, this calculation is performed as following.
Firstly, a corneal shape is assumed based on an average of a pre-operative corneal curvature obtained by a corneal curvature measurement, on the assumption that a corneal surface of an eye to be operated is spherical or toric. In the assumption, S (a spherical power), C (a cylindrical power) and A (an astigmatic axial angle) are used, the values S, C and A being obtained by a subjective refractive power measurement or an objective refractive power measurement. Then, an amount of corneal ablation is calculated on the assumption as following: a spherical surface or a toric surface which are formed by a corneal stroma is to be ablated; the values S, C and A may be corrected (or calibrated); and a post-operative corneal shape is also to be a spherical surface or a toric surface.
However, a cornea of a human eye does not always have a spherical surface or a toric surface, thus there are some cases that a corneal shape is non-symmetric. Because, a corneal surface shape is different in part due to irregular astigmatism or the like. In addition, a refractive power is not always symmetric with respect to a corneal center. In the prior art, an objective refractive power measurement apparatus only calculates the values S, C and A (these values represent a spherical surface or a toric surface), obtained by measuring a limited region within 3 mm or less from a corneal center. It is insufficient to determine an amount of corneal ablation based on the values S, C and A.
Provided that a human eye is a system of an image optics from a cornea to a retina, spherical aberration has influence upon the above described ablation that causes a post-operative corneal shape to be spherical or toric. In this point of view, the ablation in the prior art is also insufficient. Intrinsically, a corneal surface of a human eye is not spherical, there is possibility that the ablation in the prior art has bad influence upon aberration.